This invention relates generally to apparatus and processes for aerating dispersions. More particularly, the present invention relates to apparatus and processes for aerating pulp suspensions during de-inking.
De-inking flotation is a mechanical process for removing impurities and ink particles from pulp suspensions produced particularly in waste paper treatment. This process requires the generating of gas bubbles in the appropriate quantity and size distribution. Hydrophobic substances or substances to which ampholytics are added to make them hydrophobic, such as ink particles or stickies, are carried to the surface of the liquid by the gas bubbles adhering to them and can be removed from the surface as scum. This is referred to as selective flotation because the pulp is discharged with the accept due to its hydrophile nature. Processes of this type are known in numerous geometric modifications, for example from DE 41 16 916 C2 or EP 0 211 834 B1, and have reached a high technical standard. Further, it has also proved successful to use self-priming injectors to generate gas bubbles and mix these with the pulp suspension. These injectors basically comprise a propulsive jet nozzle, a mixing or impulse exchange pipe, and a diffuser. Here, the liquid flow emerging from the propulsive jet nozzle according to the open jet principle generates under pressure. As a result, gas is sucked in and mixed with the liquid as a result of the impulse exchange between liquid and gas in the mixing pipe. At the exit from the diffuser used for energy recovery a dispersion of pulp and bubbles is formed. Use of the known processes and injectors, however, has shown several disadvantages in selective flotation of pulp suspensions.
The suction effect of the known injectors in operation with pulp suspensions is too weak and the bubble size distribution generated by the injector known does not have the optimum design to meet the requirements of selective flotation.
The invention is, therefore, based on the task of designing an injector with greater suction effect and optimum bubble size distribution for use in de-inking flotation.
The process according to the invention is thus characterized by the gas, particularly air, being sucked in by the effect of the injector at a minimum of two successive points and mixed with the suspension. Due to suction taking place in stages, the pulp can be loosened by the gas in the first stage, thus achieving a better spread of the free jet in the second stage, resulting in improved suction effect and corresponding bubble generating, particularly with a reduction in the fine bubble portion to avoid solids losses.
An advantageous further development of the invention is characterized by some 20 to 95% of the entire quantity of gas, particularly air, sucked in being taken in the first stage. Since intake of the quantity of gas, particularly air, is divided over several suction points, more even mixing of the suspension with the gas is obtained. This allows a specific suitable bubble size to be set.
A favorable configuration of the invention is characterized by the gas and liquid flow obtained by suction and mixing being transferred in a free jet after the first stage. As a result, use of the kinetic energy of the jet, in particular, can be improved for renewed intake of gas.
A favorable further development of the invention is characterized by the gas or air loading of the pulp suspension directly after being sprayed in amounting to approximately 50-150%.
The invention also refers to a device for aerating dispersions, particularly a flotation device for de-inking pulp suspensions with an injector, characterized by at least two suction points being arranged in series in flow direction. Due to suction taking place in stages, the pulp can be loosened by the gas in the first stage, thus achieving a better spread of the free jet in the second stage, resulting in improved suction effect and corresponding bubble generating, particularly with a reduction in the fine bubble portion to avoid solids losses.
A favorable further development of the invention is characterized by the injection channel widening after the first suction point. Thus, the kinetic energy of the jet can be put to good use in a favorable manner.
An advantageous further development of the invention is characterized by a panel being mounted at the end of the injector channel across the flow direction. This panel acts as a radial diffuser to recover energy from the liquid jet.
An advantageous configuration of the invention is characterized by the panel being mounted on a slant to the flow direction.
A favorable further development of the invention is characterized by the panel containing internals for targeted guidance of the flow. As a result, the injector can also be mounted in any desired position in the flotation cell.
A favorable configuration of the invention is characterized by a minimum of two injectors being mounted in parallel in the form of an injector group. With this design it is also possible to handle large throughputs accordingly.